1. Field of the Invention
The present invention relates to an improved viewfinder optical system having a pentagonal roof reflecting mirror assembly and more particularly to a totally reflecting viewfinder system providing a high magnification with a relatively low distortion of the visual field for single lens reflex cameras.
2. Description of the Prior Art
The camera industry today is striving to provide lighter and more compact cameras to meet the consumer demand. At the same time, the manufacturers of cameras are being required to maintain the operating and optical capabilities of their prior cameras and where possible to improve the same. An important area for improvement within these design parameters is the viewfinder. The viewfinder is required not only to provide a relatively bright and optically acceptable image, but to further meet additional requirements of magnification and weight.
Traditionally, high quality 35 mm cameras have utilized a pentaprism to maintain the high quality optical characteristics that is expected of through the lens viewfinders. Recently in an effort to reduce the considerable weight of a pentaprism, there have been suggestions to replace it with a hollow pentagonal roof type refracting mirror assembly. While these viewfinders realize a considerable saving in weight, they have a disadvantage in that the optical path length is relatively long and the magnification of the visual field thereof is reduced in comparison with a pentaprism type viewfinder.
There have been suggestions to form a third reflecting mirror in a roof type mirror assembly into a concave spherical mirror to help eliminate the reduction in magnification and to shorten the optical path length. Problems have existed, however, in that an image inclination or tilt occurs and there is a difference in diopter power between the upper and lower portions of the visual field. These problems manifest themselves in that a lateral line in the visual field will be curved while a longitudinal line thereof will be tilted due to distortion resulting from the inherent geometry of the system.
U.S. Pat. Nos. 3,783,764 and 3,860,940 disclose a viewfinder for a foldable reflex instant camera having an aspherical concave mirror with a tilting of an ocular lens.
Of some interest is the Japanese Publication Sho49-12602 (1974) which discloses a viewfinder for a single lens reflex camera having a convex mirror to increase the viewfinder image magnification. Cited of general interest is U.S. Pat. No. 4,063,261 which discloses a concave reflecting mirror for increasing image magnification. The prior art is still seeking to provide an optimized viewfinder for modern lightweight cameras.